Current sensing field-effect transistors, commonly referred to as sensefets, have been used widely used for many years in applications where accurate current sensing can provide information for both control and over-current protection. Sensefets are typically constructed as a small part or transistor section of a larger, main current carrying semiconductor device. For example, in a conventional insulated-gate field-effect transistor (MOSFET) device, the sensefet may comprise a small section of the channel region of the main device. In operation, the sensefet may sample a small portion of the channel current of the larger device, thereby providing an indication of the current flowing through the main transistor device. The sensefet and main device typically share a common drain and gate, but each has a separate source electrode.
High-voltage, field-effect transistors (HVFETs) are also well known in the semiconductor arts. Many HVFETs (i.e., power transistors) employ a device structure that includes an extended drain or drift region that supports or blocks the applied high-voltage (e.g., several hundred volts) when the device is in the “off” state. In a prior art vertical HVFET structure, a mesa or pillar of semiconductor material forms the extended drain or drift region for current flow in the on-state. A trench gate structure is formed near the top of the substrate, adjacent the sidewall regions of the mesa where a body region is disposed above the extended drain region. Application of an appropriate voltage potential to the gate causes a conductive channel to be formed along the vertical sidewall portion of the body region such that current may flow vertically through the semiconductor material, i.e., from a top surface of the substrate where the source region is disposed, down to the bottom of the substrate where the drain region is located.
One problem that exists is that prior art sensefets designed for use with conventional MOSFETs are generally inapplicable for use with vertical power transistor structures due to the deep trench that forms the pillar isolating both the channel and drift regions.